Many contemporary inductive load drivers that provide drive current from a power supply include provision for periodically recirculating current in an inductive load. This is in order to reduce power dissipation of the driver while maintaining operative current through the load, and also to optimize the physical performance of the load. These loads may include direct current motors, electromechanical solenoids, electromechanical actuators, and other inductive loads. These loads are typically energized by a drive circuit that provides current to the particular load. Drive circuits, or drivers, come in many varieties ranging from simple solenoid drivers to complex bridge circuits. The more complex circuits not only activate and deactivate the load but control it's action precisely to overcome inherent mechanical deficiencies. Some of these drivers have the capability to recirculate current through the load to effectively prolong the drive action after the drive current provided from the power supply is removed.
Referring to FIG. 1, a prior art inductive load driver is shown. A control module 100 has an output 102 that provides a control signal 104 with a drive state 106 and a recirculation state 108. An inductive load 101 is provided drive current from a power supply terminal 105, through a transistor 103 along a drive path 109, to a second power supply terminal 111. This action is controlled by the drive state 106 of the control signal 104 provided at an input, or control terminal 107 from the control module 100.
When this path 109 is deactivated, by applying recirculation state 108 of the control signal 104 at terminal 107, the transistor 103 is turned off and a recirculating path 115 is invoked. This recirculating path 115 includes the inductive load 101 and a conducting diode 113. Because, under this circumstance, the diode 113 has a forward voltage drop, the diode 113 will dissipate power while the recirculating current is active. For small currents this is generally not a significant issue. However, with inductive loads that require larger drive currents, thereby larger recirculating currents, this power dissipation becomes a very significant problem. For instance with a recirculation current of 100 amps, and a forward voltage drop of 1.0 volts the power dissipation requirement for the diode 113 is 100 amps * 1.0 volts, or 100 watts.
The power dissipation, created by the forward voltage drop across the diode 113, will cause the diode's 113 temperature to rise significantly. This is problematic because it will cause heating of adjacent components, and significantly reduce the reliable life of the diode 113. The diode recirculation scheme also adds unnecessary component cost and detracts from available space. Additionally, since the diode 113 has a significant forward voltage drop, the recirculation efficiency is impaired.
What is needed is an improved driver for recirculating current through an inductive load that has a lower cost, a higher field reliability, a smaller size, is more efficient, and has significantly lower power dissipation.